Fuel cell technology has been identified as a potential alternative for the traditional internal-combustion engine conventionally used to power automobiles. It has been found that power cell plants are capable of achieving efficiencies as high as 55%, as compared to maximum efficiency of about 30% for internal combustion engines. Furthermore, unlike internal combustion engines, fuel cell power plants emit no harmful by-products which would otherwise contribute to atmospheric pollution.
Fuel cell stacks include three basic components: a cathode, an anode and an electrolyte which is sandwiched between the cathode and the anode. Hydrogen at the anode is converted to positively-charged hydrogen ions. These ions travel through the electrolyte to the cathode, where they react with oxygen from the air. The remaining electrons in the anode flow to an external circuit, thereby producing electricity which drives an electric motor that powers the automobile. The electrons then travel to the cathode, where they join the oxygen and the hydrogen protons to form water, thus continuing the electricity-generating cycle. Individual fuel cells can be stacked together in series to generate electricity at higher voltages.
While they are a promising development in automotive technology, fuel cells are characterized by a specific operating temperature which presents a significant design challenge from the standpoint of maintaining the structural and operational integrity of the fuel cell stack. Maintaining the fuel cell stack within the temperature ranges that are required for optimum fuel cell operation depends on a cooling system which is suitable for the purpose.
Cooling systems for both the conventional internal combustion engine and the fuel cell system typically utilize a pump or pumps to circulate a coolant liquid through a network that is disposed in sufficient proximity to the system components to enable thermal exchange between the network and the components. To achieve reliable cooling of a fuel cell, both the outlet temperature of the fuel cell stack and the change in temperature across the fuel cell stack (stack ΔT) have to be controlled. Reasonable control of both of these parameters currently requires the use of a total of six sensors: three temperature sensors, one pressure sensor, a valve position feedback sensor and a volumetric flow sensor. The use of six sensors in the fuel cell cooling system adds considerable cost to the system, especially in view of such considerations as the sensor cost; the cost of sensor wiring; the cost of connectors; the additional I/O cost in the fuel cell controller; the manufacturing cost for material handling and sensor installation; the warranty costs for replacing bad sensors; the stocking costs for maintaining spare sensors in the dealer network; the intangible cost of an additional packaging constraint; and the internal administrative costs to maintain and track additional part numbers.
In a conventional fuel cell stack cooling system, the stack outlet temperature is typically controlled primarily using a three-way radiator bypass valve that controls the quantity of heat released from the system. If the stack outlet temperature rises too high, a system controller diverts more coolant through the radiator by actuation of the radiator bypass valve to lower the temperature of the coolant. The stack outlet temperature control mechanism requires the use of two temperature sensors and a valve position feedback sensor for proper functioning.
The stack ΔT, on the other hand, is primarily controlled by the speed of the coolant pump. If the stack ΔT is too high, the system controller increases the pump speed to circulate more coolant through the fuel cell stack in order to lower the stack ΔT. In the conventional fuel cell stack cooling system, control of the stack ΔT requires the use of a coolant inlet temperature sensor, a coolant inlet pressure sensor and a volumetric flow meter. These elements are used to determine the pump speed which is needed to achieve the coolant flow rate required for optimum stack ΔT control.
The present invention is directed to a fuel cell stack temperature control system and method in which the coolant inlet temperature sensor, the coolant inlet pressure sensor and the volumetric flow meter can be eliminated from a fuel cell stack cooling system and replaced by a pump delta pressure (ΔP) sensor. The pump ΔP sensor is used in conjunction with a pump map to determine the coolant pump speed and pump ΔP which correspond to a particular coolant flow rate. The coolant pump can then be operated at the pump speed which corresponds to the coolant flow rate that is required for optimal stack ΔT control. This substitution of three sensors with a relatively inexpensive pump ΔP sensor substantially reduces the costs associated with the additional sensors which characterize the conventional fuel cell stack cooling system.